The present invention is related to calibration of test equipment and, in particular, to calibration of a digitizer in an oscilloscope, for example, using a square-wave test signal, whereby a time delay for adjustment is calculated by a microprocessor on the basis of the digitized square-wave test signal.
It is well known that a digitizer comprising, for example, a Sample-and-Hold (S/H) circuit and an analog-to-digital (A/D) converter, converts an analog signal to digital form. Due to the instantaneous changes of voltage in the input analog signal, the S/H circuit periodically samples the signal, in response to a clock signal, at particular points in time and holds (stores) a sampled voltage value to be input to the A/D converter. The A/D converter receives the sampled voltage value from the S/H circuit and obtains a digital representation based on a predetermined resolution (8 bits, 16 bits, etc.). If converted at least at the Nyquist rate, the analog signal is accurately represented by digital values and may be processed then by digital signal processing elements. Of course, the higher the frequency of the digitizer in performing the sampling, holding and converting operations as described hereinabove, the better the approximation between the input analog signal and its digital representation.
In today's fast-paced world emphasizing ever increasing speed and processing, many analog signals have frequency components which would require a very fast digitizer to convert the analog signal to digital form. Such digitizers are typically quite expensive and may be cost prohibitive for certain applications. As a result, it has been suggested to use several digitizers, whereby digital values representing the analog signal are interleaved (time-multiplexed). For example, a digitizer, operating at a frequency F/2, acquires the input analog signal at times t1, t3, t5, t7 . . . and generates respective digital values D1, D3, D5 . . . of the analog signal; while another digitizer, also operating at the frequency F/2, acquires the analog signal at times t2, t4, t6 . . . and generates digital values D2, D4, D6 . . . of the same analog signal. It is apparent that the two digitizers acquire the analog signal for conversion at different times, and specifically the analog signal is sampled by the second digitizer between the sampling operation of the first digitizer (and vice versa). If the digital values from the two digitizers are interleaved, the analog signal is represented by the digital values D1, D2, D3, D4, D5, D6, D7 . . . As a result, the effective sampling frequency of the analog signal is F, even though individual sampling frequencies of the digitizers are F/2.
Proper operation of multiple digitizers employed in the above-described interleaved manner in digital oscilloscopes, for example, depends on a very precise timing. Among other things, clock signals supplied to the digitizers should be accurately controlled to achieve a predetermined phase shift. In the above example, the conversion of the analog signal to the digital values D2, D4, D6 by the second digitizer should occur precisely between the digital values D1, D3, D5, D7 generated by the first digitizer. That is, the digitizers should be triggered with clock signals having a high accuracy. The accuracy of clock signals that generate triggers for the digitizers becomes even more important when the number of digitizers operating in the interleaved mode increases. As the interval between the interleaved samples decreases, the accuracy of the trigger clock must increase to generate accurate interleaved digital values.
In addition to clock signals, other factors such as temperature, humidity, physical characteristics of current conduits, etc. may affect the timing of the digitizers operating in the interleaved mode. In contrast to the clock signals generated by oscillators that may be manufactured with high precision albeit very expensively, the above-mentioned additional factors that influence the operation of the digitizers may be difficult to control. Periodic calibration (adjustment) of the digitizers is, therefore, necessary to obtain accurate digital values representing the analog signal. It is therefore imperative that the calibration of the digitizers should be performed quickly, accurately and without excessive hardware increasing the cost of the device or system.
A need therefore exists for a device and method for realizing the above criteria.